Materials used as non-consumable anodes in molten electrolytes must have a good stability in an oxidizing atmosphere, good mechanical properties, good electrical conductivity and be able to operate for prolonged periods of time under polarizing conditions. At the same time, the welding and machining of large components should not present unsurmountable problems to the practitioner. It is well known that ceramic materials have good chemical corrosion properties. However, their low electrical conductivity and difficulties of making mechanical and electrical contact as well as difficulties in shaping and machining these materials seriously limit their use.
U.S. Pat. No. 4,374,050 discloses inert electrodes for aluminum production fabricated from at least two metals or metal compounds to provide a combination metal compound. For example, an alloy of two or more metals can be surface oxidized to form a compounded oxide of the metals at the surface on an unoxidized alloy substrate. U.S. Pat. No. 4,374,761 discloses similar compositions further comprising a dispersed metal powder in an attempt to improve conductivity. U.S. Pat. Nos. 4,399,008 and 4,478,693 provide similar combinations of metal oxide compositions which may be applied as a preformed oxide composition on a metal substrate by cladding or plasma spraying. Such application techniques, however, are known to involve many drawbacks and the adhesion is particularly poor. U.S. Pat. No. 4,620,905 describes an oxidized alloy electrode based on tin or copper with nickel, iron, silver, zinc, magnesium, aluminum and yttrium, either as a cermet or partially oxidized at its surface. Such partially oxidized alloys suffer serious disadvantages in that the oxide layers formed are far too porous to oxygen, and not sufficently stable in corrosive environments. In addition, it has been observed that at high temperatures the partially oxidized structures continue to oxidize, and this uncontrolled oxidation causes subsequent segregation of the metal and/or oxide layer. In addition, the machining of ceramics and achieving a good mechanical and electrical contact with such materials involves problems which are difficult to solve. Adherence at the ceramic-metal interfaces is particularly difficult to achieve, and this very problem has hampered use of such simple composites. Finally, all these materials as such have not proven satisfactory as substrates for cerium oxyfluoride coatings of the type discussed below.
U.S. Pat. No. 4 614 569 describes a method of electrowinning metals by electrolysis of a melt containing a dissolved species of the metal to be won using an anode immersed in the melt wherein the anode has a metal, alloy or cermet substrate and an operative anode surface which is a protective surface coating containing a compound of a metal less noble than the metal to be electrowon; the protective coating being preserved by maintaining in the melt a suitable concentration of a species of this less noble metal. Usually, the protective anode coating comprises a fluorine-containing oxycompound of cerium (referred to as "cerium oxyfluoride") alone or in combination with additives such as compounds of tantalum, niobium, yttrium, lanthanum, praesodymium and other rare earth elements; this coating being maintained by the addition of cerium and possibly other elements to the electrolyte. The electrolyte can be molten cryolite containing dissolved alumina, i.e. for the production of aluminum.
This electrowinning method potentially has very significant advantages. To date, however, there remain problems with the anode substrate. When this is a ceramic, the conductivity may be low when the substrate is a metal, alloy or cermet, it may be subject to oxidation leading to a reduced life of the anode, despite the excellent protective effect of the cerium oxyfluoride coating which protects the substrate from direct attack by the corrosive electrolyte.
A promising solution to these problems has been the use of a ceramic/metal composite material of at least one ceramic phase and at least one metallic phase, comprising mixed oxides of cerium with aluminum, nickel, iron and/or copper in the form of a skeleton of interconnected ceramic oxide grains which skeleton is interwoven with a continuous metallic network of an alloy or intermetallic compound of cerium with aluminum, nickel, iron and/or copper, as described in EP-A-0 257 708. When used as electrode substrates, these materials have promise, particularly those based on cerium and aluminum because even if they corrode, this does not lead to corrosion products that contaminate the electrowon aluminum. Nevertheless corrosion of the substrate remains a problem.